1. Field of the Invention
The present invention relates to an apparatus for applying liquid or pasty application medium, in particular aqueous pigment suspension, to one or both sides of a moving material web, especially to a paper or board web, during its production and/or finishing
2. Description of the Related Art
Machines for the production and/or finishing of a material web, in particular a paper or board web, have widths of up to 10 m and more nowadays. In the case where the material web is coated by way of an application medium curtain, a uniform application reaching over the entire web width is to be ensured. This requirement is more difficult to realize the faster the material webs run in the machines and, in addition, also depends on the quality of the material web and the application medium.
Coating with a xe2x80x9ccurtain coaterxe2x80x9d, that is to say an application medium applicator unit, is carried out without any excess of medium. The amount of medium applied is only that which is also intended to remain on the material web. The film applied is therefore very thin.
Depending on the quality of the material web used, the formation of microscopically uncovered points therefore occurs (xe2x80x9cmicro-skip coatingxe2x80x9d), the aforementioned very thin liquid film tearing at locations with projecting paper fibers, roughness or pores. At these locations, which are different in point terms, the homogeneous contact between medium and substrate (material web) is lacking.
In order to counter this problem, one solution for the electrostatic charging of the material web and/or the application medium and/or the applicator unit has been proposed in DE 100 12 256. DE 100 57 731 is likewise concerned, among other things, with the problem of the more uniform wetting of the material web to be coated by a curtain applicator unit. In the case of this solution, the arrangement of a vacuum generating device in the form of a suction box is provided in the pocket between the application medium curtain and the material web. In the case of direct application to the material web, the vacuum is intended to promote anchoring of the application medium on the material web. In order to increase its efficiency, the aforementioned suction box is equipped with doctor elements, which are set against the material web, on the inlet side and/or the outlet side (in relation to the direction of the moving material web).
What is needed in the art is a suitable apparatus to improve further the adhesion to a moving material web of the application medium curtain produced by a curtain applicator unit, and to ensure still more uniform wetting of the material web surface.
The present invention provides a curtain application unit and a vacuum generating device which attracts the application medium to a moving material web.
By producing a vacuum between the already largely dried material web and the wet layer of the application medium, in spite of an inhomogeneous surface of the material web, entirely uniform wetting with application medium can be achieved, so that all the particles, irregularities, pores and the like on or in the material web are reliably covered with application medium.
The vacuum generated and applied to the underside (that is the uncoated side, facing away from the application medium curtain) of the material web needs, however, only to be sufficiently great that the application medium merely rests on the surface of the material web, but does not penetrate the latter.
As a result, the material web becomes more stable and therefore tends far less to the tearing-in effect, which is particularly desirable in the case of webs containing waste paper.
As distinct from a solution for coating a still wet paper or board web running on wires, according to DE 198 23 738, in the case of the solution according to the present invention, no moisture needs to be sucked out of the web. The vacuum only needs to be high enough that it accelerates the adhesion of the application medium to the substrate (that is to say, the material web). It is therefore possible to manage with a very low suction output from the vacuum generating source.
An expedient refinement of the apparatus can consist in the vacuum generating device being constructed as a machine-width, external suction box, which can be fitted under the material web in a straightforward manner and, in addition, may be arranged at the level or at least in the region of the line of incidence or position of incidence P of the application medium curtain. In this manner, the effect with regard to adhesion and wetability may be increased considerably as compared with DE 100 577 318 cited previously.
By way of the sealing elements arranged on the suction box, for example in the form of doctors, brushes or the like set against the material web, the suction action and therefore the adhesion of the curtain can be increased.
For configurations in which the material web is to be coated in the supported state, the support can be provided with a roll or another supporting element. This supporting element can be equipped with a suction zone which has individual suction openings and can be connected to a vacuum generating source.
The apparatus according to the present invention may be used both as an individual apparatus and in combination with the aforementioned solutions.
With the apparatus according to the present invention, sensitive, less strong papers, those with a low grammage or else with a low raw-material quality can be treated particularly advantageously, since there is no stress involved in the application of the medium, but nevertheless good wetability is provided.
Following the treatment with the apparatus according to the present invention, further conventional treatment steps, such as further coatings to produce a multiple coating, with and without intermediate drying, calendering and so on may follow.
For completeness, mention should be made of the fact that the web can subsequently be coated on the second side as well. However, the apparatus according to the present invention is then less effective, since the suction action is then reduced because of the coating of the first web side. In the case of paper webs with a low grammage, however, this should be feasible.